This invention relates to an apparatus and method for measuring the thickness of a thin film layer on a sample plate, and measuring the refractive index of the material of the thin film layer as a function of wavelength.
The optical properties of thin films are often sensitive to the process used for producing the film. For example in thin film deposition the index of refraction is often sensitive to partial pressures of process gases, beam voltages, substrate temperatures, and a host of other parameters, some of which are not even well known or understood. Complex multi-layer thin film designs require accurate knowledge of the optical properties of the individual thin film layers. Manufacturers therefore have a need to accurately measure the optical properties of their thin film layers from a particular process. The properties of special interest are the refractive index and the extinction coefficient (or the real and imaginary parts of the complex index), both of which depend on wavelength. Currently they do this using either an ellipsometer, a prism coupling system, or by a standard method for extracting the parameters from transmission spectra for a single-layer film.
Extracting the index from the transmission spectrum is usually the least expensive method, because thin film manufacturers usually already have a spectrophotometer (or many) for production control of their films. This method has the advantage that it is applicable to all wavelengths accessible to the spectrophotometer, but it has the disadvantage that it is traditionally not very accurate.
Prism couplers and Ellipsometers are quite expensive and are usually more limited in the range of wavelengths over which the index can be measured.
The difficulty with measuring the index of refraction for thin film materials from a transmission scan is that the gross features in the transmission spectrum are largely dependent on only the optical thickness, which is the product of the index and the thickness, but the gross spectral features are not as sensitive to the index and the thickness individually. Thus the optical depth can be determined easily and accurately, but it is difficult to measure the thickness and the index individually. Another way to describe this situation is that a fractional change in the index or thickness has largely the same effect on the spectrum, so it is difficult to determine index and thickness individually when fitting a transmission spectrum to extract these parameters. There are some differences, mainly in the transmission levels, but these are more subtle and difficult to measure accurately.
The situation changes radically when incident angles are beyond the critical angle. Here the phases become a sensitive function of the index. Because the phase changes are sensitive to the index, the gross features of the spectrum now change differently when the index and thickness are changed by the same fractional amount. This is the essential physics involved in using prism coupling to measure the index of refraction.